Datasheet

LT3686A
11
3686afa
ApplicAtions inForMAtion
Figure 3. Continuous Mode Operation Near Minimum On Time
When the required on time decreases below the typical
minimum on time of 100ns, instead of the switch pulse
width becoming narrower to accommodate the lower duty
cycle requirement, the switch pulse width remains fixed at
100ns. The inductor current ramps up to a value exceed-
ing the load current and the output ripple increases. The
part then remains off until the output voltage dips below
the programmed value before it begins switching again
(Figure 4).
Figure 4. Pulse Skip Occurs When Required On Time Is
Below 100ns
The maximum input voltage is determined by the absolute
maximum ratings of the V
IN
and BOOST pins. For fixed
frequency operation, the maximum input voltage is de-
termined by the minimum duty cycle DC
MIN
:
V
IN(MAX)
=
V
OUT
+
V
D
DC
MIN
– V
D
+ V
SW
DC
MIN
can be adjusted with frequency. Note that this is a
restriction on the operating input voltage for fixed frequency
operation; the circuit will tolerate transient inputs up to the
absolute maximum ratings of the V
IN
and BOOST pins.
Minimum On Time
As the input voltage is increased, the LT3686A is required
to switch for shorter periods of time. Delays associated
with turning off the power switch dictate the minimum on
time of the part. The minimum on time for the LT3686A
is 100ns (Figure 3).
V
IN
= 18V
V
OUT
= 3.3V
L = 6.8µH
C
OUT
= 22µF
I
LOAD
= 1.2mA
V
OUT
100mV/DIV
AC
V
SW
10V/DIV
I
L
500mA/DIV
500ns/DIV
3686A F03
V
IN
= 35V
V
OUT
= 3.3V
L = 6.8µH
C
OUT
= 22µF
I
OUT
= 300mA
V
SW
20V/DIV
I
L
500mA/DIV
V
OUT
100mA/DIV
AC
2µs/DIV
3686A F04
Provided that the load can tolerate the increased output
voltage ripple and that the components have been prop-
erly selected, operation while pulse skipping is safe and
will not damage the part. As the input voltage increases,
the inductor current ramps up quicker, the number of
skipped pulses increases, and the output voltage ripple
increases.
Inductor current may reach current limit when operating
in pulse skip mode with small valued inductors. In this
case, the LT3686A will periodically reduce its frequency